This invention relates to components and processes for fiber optic related component fabrication. More particularly, the invention relates to the fabrication of optical coupling and waveguiding elements.
Optical Fibers in commercial systems have been traditionally held by using a combination of pieces.
A connector assembly 100, such as shown in FIG. 1 as an exploded view is used to attach various fiber pieces (or fiber pieces and modules) together. A ferrule 102 is the part of the connector 100 into which the fibers 104 themselves are inserted before the ferrule 102 is inserted into the overall connector itself. The ferrule 102 is a xe2x80x98high-precisionxe2x80x99 piece of the assembly 100. It holds the fiber(s) 104 in a precise position and ensures that when two connector pieces are attached, that the fibers in the two pieces are held in accurate alignment. The remainder of the connector 106 is xe2x80x98low precisionxe2x80x99 relative to the ferrule 102.
In the multi-fiber connectors available today, most of the connections are for fiber arrays of 2 or more fibers, such as shown in U.S. Pat. No. 5,214,730, up to arrays of 1xc3x9712 (although some commercial 2xc3x9712 configurations have been tried). The connectors employed are referred to by various names depending upon who makes them. In 1xc3x972 arrays, connectors are referred to as ST, LC, MT-RJ connectors while for 1xc3x9712 arrays the connectors are referred to as MTP(copyright), MPO, MPX and SMC connectors, among others. In the 1xc3x9712 or 2xc3x9712 area, all of the various connectors use a common type of ferrule commercially available from, among others, US Conec Ltd. and Alcoa Fujikura Ltd. In addition, commercial connectors for small arrays (less than 12) fibers have also been proposed, for example, in U.S. Pat. No. 5,743,785.
Fiber holding pieces, such as ferrules 102, can be made by molding plastic or epoxy pieces containing holes 108 into which optical fibers 104 can be inserted. Fibers must be able to be centered in each hole precisely and repeatably.
When an array of holes is made in a material for holding optical fibers, there are two aspects which need to be controlled. The spacing between holes (the xe2x80x9cpitchxe2x80x9d of the holes) and the diameter of each hole. Both have some margin of error due to the inherent inaccuracies of the fabrication techniques. If inaccuracies introduce errors in either (or both) pitch or size that are too large, then the fibers can be inserted at an angle or will not be positioned correctly in the ferrule. In either case, this negatively affects the ability to couple light efficiently, if at all, from one bundle to another or from an optical or opto-electronic component to a fiber bundle. If the hole pitch is inaccurate, then fibers from one bundle will not line up well with fibers of another bundle. However, even if the center-to-center pitch of the holes is very accurate, because the hole diameter is larger than the fiber (and each hole likely varies across an array) each fiber need not be in the exact same place in the hole as the other fibers in their holes, then that can cause misalignment, leading to inefficiencies or unacceptable losses. For example, if each of the holes in a ferrule piece was accurate to within 4 microns, then adjacent fibers could be off in pitch by up to 4 microns, since one fiber could be pushed to one side by 2 microns and the adjacent fiber could be pushed in the other direction by 2 microns. While this may be acceptable for multi-mode fibers, for single mode fibers this would be a huge offset that could make connections unacceptable or impossible.
In addition, fibers should generally not be placed in a hole at an angle or, if inserted at an angle, the particular angle should be specifically controlled.
FIG. 2 shows an example connector hole 200 and fiber 202. The inner circle, represents an actual fiber 202 while the outer circle, represents the hole 200 in the ferrule. As shown, the difference in sizes is not to scale but is exaggerated for purposes of illustration. Nevertheless, in actuality, the ferrule hole 200 must be larger than the fiber 202 by enough of a margin to allow for easy insertionxe2x80x94ultra-tight tolerances can not be effectively used. While the fiber 202 should ideally be centered with respect to the hole 200, as can be seen in FIG. 3, any individual fiber 202 could also be pushed in any hole 200 to somewhere else in the hole, for example, either the left or right edge (or any other edge) where it would not be centered within the hole 200. Thus, even if the ferrule has an accurate pitch xe2x80x9cPxe2x80x9d between hole centers 204, adjacent fibers 200 in an array may have an incorrect pitch xe2x80x9cP+2xcex94Pxe2x80x9d due to the offset xcex94P between the center 206 of each hole 200 and where the fiber 200 lies within the hole 200, in this case, causing an incorrect pitch of P plus 2 times the individual offset xcex94P in each hole.
The 1xc3x9712 and 2xc3x9712 ferrule technology currently in commercial use is based upon a glass filled epoxy resin (a high-performance plastic) which is fabricated using a common plastic molding technique called transfer molding. Today, ferrules molded out of epoxies or plastics can be made to the necessary tolerances for multimode fibers, but special care must be taken during fabrication. Plastic molding technology is very process sensitive and molds having the requisite precision are extremely difficult to make. Even so, yields tend to be poor due to the inherent manufacturing process errors that occur in plastics molding. Since the tolerances on these pieces must be very accurate (on the order of about 1 to 2 micrometers), high yield manufacture is difficult. As a result, the cost of terminating fiber bundles into these connectors can be quite expensive, running hundreds of dollars per side. In addition, the process is not scalable to larger numbers of fibers (particularly 30 or more) because of inaccuracies and yield issues associated with molding technology and reliable production of ferrules for similar numbers of single mode fibers is even more difficult.
There has been an increasing need among users in the fiberoptic field for larger groups of fibers, so demand for connectors to handle these groups has been increasing as well. As a result, creation of connectors for larger arrays, such as 5xc3x9712, have been attempted. One manufacturer is known to have made a 5xc3x9712 connector array, but achieved such poor yields that they deemed an array of that size unmanufacturable. Moreover, the cost of producing the pieces resulted in their being sold for $500 each, due to poor yield, and the mold for producing the pieces was destroyed during the process.
The problem is that in plastic molding pieces for holding higher fiber counts in small spaces results in less structural integrity for the molded piece. As such, the prior art has been forced to do without commercial connectors for such large arrays, because 5xc3x9712 arrays can not be reliably created and commercial connectors for larger format arrays (e.g. even a 6xc3x9712) are considered prohibitively difficult to even attempt.
The ferrule area is very small, since ferrules for the above MTP, MPO, MPX or SMC connectors are about 0.07xe2x80x3 high, 0.3xe2x80x3 wide and 0.4xe2x80x3 deep, so molding or machining of features in the ferrules of the sizes required to hold multiple optical fibers (which typically have about a 125 micron diameter for a multimode fiber and a 9 micron diameter core for a single mode fiber) is very difficult. Since single mode fibers have an even smaller diameter than multimode fibers, molding or machining ferrules to accommodate large arrays of single mode fibers is currently, for all practical purposes, impossiblexe2x80x94particularly on a cost effective commercially viable scale.
Additionally, making ferrules for arrays is made more difficult due to process variations during production because, as the holes approach the edge of the ferrule, the structural integrity of the walls decrease causing parts to have poor tolerance at the periphery, become overly fragile causing component collapse in some cases, or prohibiting removal of material from the inside of the piece that impedes or prevents fiber insertion.
Some have attempted to make two-dimensional fiber bundle arrays for by creating a dense packing of fibers together, for example, as described in U.S. Pat. No. 5,473,716, and K. Koyabu, F. Ohira, T. Yamamoto, xe2x80x9cFabrication of Two-Dimensional Fiber Arrays Using Microferrulesxe2x80x9d IEEE Transactions on Components, Packaging and Manufacturing Technologyxe2x80x94Part C, Vol 21, No 1, January 1998. However, these attempts have not yielded a solution, particularly for the types of connectors mentioned above, because the inaccuracies of fiber production result in diameters of fibers which fluctuate within a 2 micron range (i.e. plus or minus 1 micron). Hence if 12 fibers are stacked in a row, there could be as much as 12 microns of inaccuracy in fiber alignment. Even with multi-mode fibers (the best of which use 50 micron cores), a misalignment of 12 microns will cause unacceptable light loss for most applications. For single mode fibers, which typically have 9 micron diameter cores, a 7 to 12 micron misalignment could mean that, irrespective of the alignment of the fiber at one end of the row, entire fibers at or near the other end of the row could receive no light whatsoever. For two-dimensional fiber arrays, the problem is even worse because the inaccuracy of the fiber is not limited to one direction. Thus, for example with a 16xc3x9716 array, a plus or minus 1 micron inaccuracy could result in fiber misalignments by up to 23 microns or more. Compounding the problem is the further fact that fiber inaccuracies stated as plus or minus 1 micron do not mean that fiber manufacturers guarantee that the fiber will be inaccurate by no more than 1 micron. Rather, the inaccuracy statement represents a standard deviation error range. This means that most of the fiber should only be that inaccurate. Individual fibers, or portions thereof, could have larger inaccuracies due to statistical variations.
As a result, the larger the number of fibers, the more likely a problem due to fiber inaccuracy will occur because, for example, using the 16xc3x9716 array above, the array would have 256 times the chance (because there are 16xc3x9716=256 fibers) of having at least one of these statistically anomalous fibers in the group.
Others have attempted to align two dimensional arrays of fibers (e.g. 4xc3x974 arrays) in a research setting, but none have applied their techniques to conventional connector technologies. Moreover, the techniques are not suitable or readily adaptable for high yield, low cost, mass production as demanded by the industry. For example, some groups have examined the use of micromachined pieces made out of polyimide as described in J. Sasian, R. Novotny, M. Beckman, S. Walker, M. Wojcik, S. Hinterlong, xe2x80x9cFabrication of fiber bundle arrays for free-space photonic switching systems,xe2x80x9d Optical Engineering, Vol 33, #9 pp. 2979-2985 September 1994.
Others have attempted to use silicon as a ferrule for precisely holding fiber bundle arrays since silicon can be manufactured with very high precision (better than 1 micron) and techniques for processing of silicon for high yield is, in general, well understood.
Early attempts at silicon machining for two-dimensional array fiber placement were performed with some limited success and one-dimensional fiber arrays, using fibers placed in V-Grooves etched into a piece of silicon, have been created, for example, as shown in FIG. 4A. The approach used the silicon pieces to hold the fibers but no attempt was made to integrate such an arrangement into a commercial connector.
Other groups took the V-Groove approach of FIG. 4A and performed an experiment where they stacked two of pieces together FIG. 4B for insertion into a connector. This resulted in a minimal array with two rows of fibers, as described in H. Kosaka, M. Kajita, M. Yamada, Y. Sugimoto, xe2x80x9cPlastic-Based Receptacle-Type VCSEL-Array modules with One and Two Dimensions Fabricated using the self-Alignment Mounting Technique,xe2x80x9d IEEE Electronic Components and Technology Conference, pp. 382-390 (1997), but the technique was not scalable to larger format two-dimensional arrays, such as shown in FIG. 4C.
Still other groups looked at holding larger format two-dimensional arrays using silicon pieces machined using wet-etching techniques, as described in G. Proudley, C. Stace, H. White, xe2x80x9cFabrication of two dimensional fiber optic arrays for an optical crossbar switch,xe2x80x9d Optical Engineering, Vol 33, #2, pp. 627-635 , February 1994.
While these silicon pieces were able to hold fibers, they were not designed to be, and could not readily be, used with existing ferrule or connector technology. Moreover, they could not be used for single mode fibers with any accuracy.
Thus, none of the above attempts have provided a viable solution to the problem of how to effectively create a large format fiber array which: allows for high precision holding of large arrays of fibers, especially single mode fibers, is compatible with current commercially used connectors that attach two fiber bundles to each other or one fiber bundle to a component containing an array of optical devices, such as lasers and/or detectors, and that allows for easy fiber termination in a rapid fashion at low production cost.
In addition, because of the above problems, there is presently no large format ferrule apparatus that can maintain fibers at a low angle, or at a precisely specified angle, for good optical coupling.
Collimating arrays are conceptually arrays of pipes for light. Mass production of collimating arrays for commercial applications has largely been dominated by the digital photographic camera and digital video camera world. These applications typically use a device called a xe2x80x9cfaceplatexe2x80x9d, which is a multi-fiber assembly used to direct light onto optical detectors used for imaging. Since, for cameras, effective imaging requires the maximum amount of light reach the detectors, a faceplate will have several fibers per individual detector. In fact, in the most desirable faceplates, the number of optical fibers exceeds the number of optical detectors by many times. Thus, light being directed to a single detector in such a camera passes through multiple optical fibers arranged in parallel, and a camera has one detector per pixel. For imaging systems like cameras, this collimating technique is sufficient to accomplish its purpose. However, when dealing with optical communication systems, faceplates can not be used because the light loss resulting from such a collimating arrangement is significant. The faceplate technique (sometimes also referred to as oversampling) is also incompatible with the use of single mode fibers or lasers (which are highly desirable for use in high speed, long distance data transmission). Hence, the collimating technique of using a faceplate, such as made for use in cameras, is an unworkable approach for opto-electronic communication devices.
As noted above, for one-dimensional optical device arrays, attempts have been made to create collimators by using a piece of silicon wafer, into which V-Grooves are etched, and laying the fibers into the V-Grooves as shown in FIG. 4A. This is an operational approach for forming a one-dimensional array that is unsuitable for mass production.
Other groups have attempted to stack multiple V-Groove arrays on top of one another (FIGS. 4b, 4c) to create a larger collimating element. Unfortunately, the accuracy of stacking in the second dimension is limited by the accuracy of the thickness of the individual wafers, both on an absolute basis and on a relative basis, due to thickness variations over the area of the wafer. In addition, the stacked V-Groove technique requires such accuracy that individual stacks must be individually built up one at a time; a costly and inefficient process.
Similarly, optical waveguides are also conceptually pipes for light. Presently, there are also no inexpensive two dimensional optical waveguide combiners available for commercial applications or that can be used with a fiber array. In some cases, optical fibers are twist fused to form a 2 to 1 xe2x80x9cYxe2x80x9d branch, for example, for coupling a pumping laser to a single, signal carrying, fiber. For one-dimensional arrays of devices, Y branches have been created on the surface of a wafer by patterning, using lithographic techniques, to form waveguides. This technique provides robust control for a one-dimensional array, but cannot be extended into two dimensions since it is inherently a planar process.
Other methods for making structures for guiding light center around a field known as xe2x80x9cphotonic integrated circuitsxe2x80x9d and approaches for making them fall into three general classes.
The first class, shown in FIG. 31, involves patterning waveguides 3102 on top of a substrate 3104. By way of example, the waveguides 3102 can be polyimide and the substrate 3104, glass. The problem with this approach is that it is not applicable for 2-dimensional array formatting since the intended height of the waveguides 3102 can be as much as 30 microns, but must have sub-micron tolerance and uniformity across the substrate 3104. For mass production, this typically means across an 8 inch or larger wafer. Obtaining this level of accuracy is prohibitive if not impossible to achieve for waveguides 3102 patterned above the substrate 3104.
The second class, shown in FIG. 32, involves defining waveguides 3202 within a substrate 3204 using an implant or irradiation technique to change the refractive index of the substrate 3204 in various regions. The problem with this approach is that the typical refractive index change between the implanted or irradiated region is a gradient that is so small relative to the substrate that unacceptable levels of light leakage can occur at bends, turns or tapers in the structure. Thus, this approach is poorly suited for waveguides that are not straight.
A hybrid approach, shown in FIG. 33, using a combination of the first and second class approaches, defines regions 3302 in the substrate 3304 by implant or irradiation and uses pattern etches 3306 on top to bound the light. However, the same loss problems typical of the second class of processes occur. In addition, most substrates that would be used in an etch process, such as in the first and hybrid approaches, are glasses or crystals which are difficult to etch to significant depths, for example, 30 microns or more, with an accuracy of 1 micron or less.
A third class uses voltage to define waveguides. However, this class similarly has problems typical of those occurring with the second class of processes. In addition, this class has the further disadvantage of requiring the application of electric power to define the regions, which is highly undesirable.
Thus, there remains a need in the art for high accuracy, low loss waveguides or couplers that can be manufactured on a commercial production scale.
We have created a processing and fabrication technique for multi-piece ferrule technology that satisfies the different needs in the art. With our approach, oxidation of a wafer, typically silicon, is used to create a cladding region on the wall surfaces of a desired light path, called a xe2x80x9cframexe2x80x9d, to be formed in a coupling or guiding structure that has a low refractive index relative to the material that will be used to fill the light path. The process prevents the absorption of light in, and leakage of light to, the silicon frame in which guiding structures are made. By applying the teachings herein, fabrication of optical coupling and waveguiding elements according to a simple, but highly accurate, processing scheme is made possible. Moreover, these optical coupling and waveguiding elements exhibit extremely low loss of light through the structures, particularly where the light path includes bends, turns or tapers.
Advantageously, the technique is scalable, permitting concurrent manufacturing of multiple such devices on individual wafers, irrespective of wafer diameter, the only limitations being the due to number and size of the devices that will fit within a wafer""s area and/or the number of wafers that can be concurrently etched and/or oxidized. Such limitations however, are independent of the invention.
By using our approach, optical coupling and waveguiding elements can be made at a lower material cost, in a highly accurate manner, on a production scale previously unavailable, and in a manner that is not overly labor intensive.
Moreover, the technique allows the creation of optical elements that provide additional benefits because they can be fit into a connector, may or may not hold optical fibers, and can add a third dimension of freedom. This enables the construction of not only fiber holding elements, but also collimator arrays, Y branch, two-dimensional waveguides, and three-dimensional optical integrated circuits.
One aspect of the invention involves a method of forming a guide for light in a high refractive index material. The method involves forming a guiding structure, having a wall defining a cavity, into a surface of the high refractive index material, treating the high refractive index material with a reactive gas, by exposing the wall to the reactive gas, to cause the wall to become a cladding material having a relatively low refractive index, and, after the treatment, filling the cavity with an optically transparent material having a refractive index sufficiently above that of the cladding so that light introduced into the optically transparent material will be directed along the guiding structure.
Another aspect of the invention involves a light guide manufactured using some of the described methods.
A further aspect of the invention involves a light guiding device having a slab, including a first surface, a second surface, and a high refractive index, a guide, having a first end and a second end, the guide being located within the slab and disposed between the first surface and the second surface, the guide having a wall surface covered with a material, derived from the slab, having a first refractive index, the first refractive index being lower than the high refractive index, the wall surface defining a cavity within the slab, and, a filler material, within the cavity, and having a second refractive index sufficiently higher than the first refractive index such that light entering the first end of the guide will be directed towards the second end.
These and other aspects described herein, or resulting from the using teachings contained herein, provide advantages and benefits over the prior art. For example, one or more of the many implementations of the inventions may achieve one or more of the following advantages or provide the resultant benefits of: ease of insertion into a large format ferrule, high yield, low cost assembly, high precision, design scalability, application scalability, integration into standard commercial connectors, compatibility with commercial connector through-hole pin-placement, manufacturability in a mass-production wafer scale process, compatibility with the thermal coefficient of expansion of silicon chips used for transmission and reception of data, lower material cost, lower labor cost, high two- and three-dimensional accuracy (since etches can be placed with lithographic precision and oxidation further increases this precision), pieces can be stacked arbitrarily and/or large numbers to make waveguides which change in two- or three dimensions along their length, collimated couplers, optical routers, etc . . . , individual wafer thickness is irrelevant so cheaper, less controlled material can be used, stacking on a wafer basis rather than on a piece basis to allow for integration on a massive scale.
Additional advantages achievable in some variants include: the ability to easily create highly accurate two-dimensional and three-dimensional light directing structures inexpensively, through the use of commercially available silicon wafers since silicon wafers of exact thickness are widely available; ease of manufacture, since patterning and etching of silicon can be accomplished to very accurate sizes and depths; wafer scale manufacturability, because the processes used are all compatible with current wafer scale fabrication techniques; high precision, because opening sizes or other features can be controlled to sub-micron accuracy; and, creation of very high-confinement optical structures, having smooth sidewalls of a highly uniform, extremely controllable refractive index material, so that almost all light entering the resultant guide structure will be transmitted through it.
The advantages and features described herein are a few of the many advantages and features available from representative embodiments and are presented only to assist in understanding the invention. It should be understood that they are not to be considered limitations on the invention as defined by the claims, or limitations on equivalents to the claims. For instance, some of these advantages are mutually contradictory, in that they cannot be simultaneously present in a single embodiment. Similarly, some advantages are applicable to one aspect of the invention, and inapplicable to others. Thus, this summary of features and advantages should not be considered dispositive in determining equivalence. Additional features and advantages of the invention will become apparent in the following description, from the drawings, and from the claims.